How to handle files from different MS-versions?

Hi - and thanx lot for MuseScore! I'm working with it for years,
the most in version 2.0.2 under Ubuntu.
Sorry, mein erster Post - extra sehr einfach und auf englisch,
weil ich das englische Forum erwartet hatte... Aber müsste
zu verstehen sein? ;)
Now i've installed MS3 on an airbook and made a few files
but cannot open them in 2.0.2.
So I have two simple questions:
1) Is there any way to "import" or "convert" them for
use in this lower version as well?
2) And if I upgrade the older version to 3,
will all my older file still work fine?
Thank you for support,
best regards, musol